字符串转数组 首先想到的是split 方法 

let str = '1234567' 
console.log(str.split('')) // ["1", "2", "3", "4", "5", "6", "7"]
let str1 = '1,2,3,4,5,6,7' 
console.log(str1.split(',')) // ["1", "2", "3", "4", "5", "6", "7"]

后来想到了 Array.from() 

用法如下：

let str1 = '123567' 
console.log(Array.from(str1)) // ["1", "2", "3", "5", "6", "7"]

但是！！

let str1 = '1,2,3,' 
console.log(Array.from(str1)) // ["1", ",", "2", ",", "3", ","]

接下来，谈谈Array.from()

Array.from()方法就是将一个类数组对象或者可遍历对象转换成一个真正的数组。(类数组对象？所谓类数组对象，最基本的要求就是具有length属性的对象。)

let obj = {0:'a',1:'b',length:2}
console.log(Array.from(obj))   // ["a", "b"]

let obj1 = {0:'a',1:'b',length:3} // length属性是可读可写的
console.log(Array.from(obj1)) ["a", "b", undefined]

let obj2 = {0:'a',1:'b',length:1}
console.log(Array.from(obj2)) // ["a"]

let obj = {0:'a',1:'b'}
console.log(Array.from(obj))   //[]

let obj = {
    'name': 'tom', 
    'age': '65',
    'sex': '男',
    'friends': ['jane','john','Mary'],
    length: 4
}
console.log(Array.from(obj)) // [undefined, undefined, undefined, undefined]

let obj = {
    0: 'tom', 
    1: '65',
    2: '男',
    3: ['jane','john','Mary'],
    length: 4
}
console.log(Array.from(obj)) // ["tom", "65", "男", Array(3)]

总结：

　1、该类数组对象必须具有length属性，用于指定数组的长度。如果没有length属性，那么转换后的数组是一个空数组。

    2、类数组对象的属性名必须为数值型或字符串型的数字